Memory management fixed.

Issue #31
I removed all `create' methods, and now each expression copies all
the objects passed to its constructor.
Now Query accepts a reference in its constructor instead of a pointer.

Issue #23
Tests modified according to new structure of experssions.

Issue #23
Destrcutor of Expr was protected. Changed to Public.

Issue #13
Structure of Expressions changed in libtocc. CLI changed respectively.

Issue #13